Many commercially useful microorganisms use glucose as their main carbohydrate source. However, a disadvantage of the use of glucose by microorganisms developed for production of commercially desirable products is the high cost of glucose. The use of sucrose and mixed feedstocks containing sucrose and other sugars as carbohydrate sources for microbial production systems would be more commercially desirable because these materials are usually readily available at a lower cost.
A production microorganism can function more efficiently when it can utilize any sucrose present in a mixed feedstock. Therefore, when a production microorganism does not have the ability to utilize sucrose efficiently as a major carbon source, it cannot operate as efficiently. For example, bacterial cells typically show preferential sugar use, with glucose being the most preferred. In artificial media containing mixtures of sugars, glucose is typically metabolized to its entirety ahead of other sugars. Moreover, many bacteria lack the ability to utilize sucrose. For example, less than 50% of Escherichia coli (E. coli) strains have the ability to utilize sucrose. Thus, when a production microorganism cannot utilize sucrose as a carbohydrate source, it is desirable to engineer the microorganism so that it can utilize sucrose.
Recombinant bacteria that have been engineered to utilize sucrose by incorporation of sucrose utilization genes have been reported. For example, Livshits et al. (U.S. Pat. No. 6,960,455) describe the production of amino acids using Escherichia coli strains containing genes encoding a metabolic pathway for sucrose utilization. Additionally, Olson et al. (Appl. Microbiol. Biotechnol. 74:1031-1040, 2007) describe Escherichia coli strains carrying genes responsible for sucrose to degradation, which produce L-tyrosine or L-phenylalanine using sucrose as a carbon source. Eliot et al. (U.S. Patent Application No. 2011/0136190 A1) describe recombinant bacteria that produce glycerol and glycerol-derived products from sucrose.
However, problems remain in engineering production microorganisms so that they can utilize sucrose effectively. Specifically, high levels of expression of sucrose transport genes result in poor growth on sucrose because excess sucrose transport is inhibitory. On the other hand, low levels of sucrose transport also result in sub-optimal growth on sucrose. Therefore, it is difficult to obtain the proper sucrose transporter gene expression level. Additionally, expression of sucrose transport genes under conditions at which sucrose transport is in excess, such as at high sucrose concentrations, may inhibit growth even at gene expression levels at which growth is not inhibited at lower sucrose concentrations. Therefore, a need also exists for a sucrose transporter that can enable growth on sucrose over a broad range of sucrose concentrations.